The present invention relates to a method for transmitting at least one first and one second data signal in polarization division multiplex in an optical transmission system which includes a transmitting arrangement and a receiving arrangement which are connected to one another via at least one optical fiber link section.
In optical transmission systems, the transmission capacity of existing optical transmission systems can be extended by transmitting the optical data signals in polarization division multiplex. To transmit optical data signals in polarization division multiplex, in each case two carrier signals which are respectively modulated with a data signal are generated in at least one transmitting arrangement with the same wavelength. The first and second modulated signals exhibit a mutually orthogonal polarization in this case. The mutually orthogonally polarized modulated signals are combined to form an optical polarization division multiplex signal. The optical polarization division multiplex signal is injected into the optical transmission fiber and transmitted via the optical transmission link to a receiving unit. At the receiving end, the two orthogonally polarized modulated signals are recovered from the polarization division multiplex signal in a wavelength-dependent and polarization-dependent manner.
One of the essential problems in transmitting optical data signals in polarization division multiplex is separating the two orthogonally polarized modulated signals precisely out of the polarization division multiplex signal. This requires the determination of a control criterion for controlling a polarization control element arranged at the receiving end from the received optical polarization division multiplex signal. The modulated signals transmitted in a mutually orthogonally polarized manner are separated with the aid of the controlled polarization transformer and, for example, a downstream polarization splitter or a polarization filter.
To control the separation of the two orthogonally polarized modulated signals at the receiving end, different control criteria are known. From the publication “Optical polarization division multiplexing at 4GB/S” by Paul M Hill et al., IEEE Photonics Technology Letters, Vol. 4, No. 5, May 1992, the use of coherent techniques in combination with pilot tones for reconstructing or, respectively, separating the polarization division multiplexed optical signals is known. Furthermore, separating the polarization division multiplexed optical data signals via a correlation signal generated from the recovered clock and from the received optical signals is known from the publication “Fast Automatic Polarization Control System”, Heismann and Whalen, IEEE Photonics Technology Letters, Vol. 4, No. 5, May 1992.
Moreover, a method for separating the two signals transmitted orthogonally polarized at the receiving end via the evaluation of the autocorrelation function of one of the signals transmitted is known from international application WO 01/65754 A1. In this arrangement the control of the polarization control element at the receiving end is determined with the aid of the extinction ratio of the respective transmitted signal, calculated via the autocorrelation amount.
An object of the present invention is to devise a novel method and optical transmission system for transmitting high-bit-rate optical signals in polarization division multiplex.